If HG failure was causing the cooling system to pressurise then I guess the coolant hoses would go hard with the engine running, even stright after statrting from cold? Could be worth checking this. Also get a 'sniff test' done on coolant in the reservoir with the engine running, to check for exhaust gases, which would confirm HG failure.

Best to try to confirm the diagnosis before assuming the worst.

As an alternative, could it just be a leaking pressure cap on the coolant reservoir? This can leak when hot but show no evidence of leakage when cold. Is there any sign of coolant escaping from any of the hoses / hose joints or from the reservoir itself (i.e: dried deposits of the same colour as the coolant)?

b*****!

any idea how much it will roughly be? and if i keep topping it up like i have been doing, will it do any damage??